Your organisation must be registered in a national business/trade register, generally managed by the national “Chamber of Commerce and Industry”. Please check your documentation to find such number, but many organisations mention it even in official paper (e.g. in the header of the footer).
If you organisation is of type “limited liability”, you may easily find the information by consulting the BRIS portal. Here you can verify, and for ANY company type, also the Business register identifier and the EUID.
A list of the Business/trade registers in EU countries is available as well from the BRIS website.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.